A site is a location where network elements or other telecommunication equipment have been installed. A site typically comprises, for example, a base station and transmission equipment, with an equipment shelter and antenna tower. Due to the costs related to site acquisition and site maintenance, traditionally an effort has been made to locate network equipment that function on different frequency bands on one site. Recently the possibility to co-site also base stations of different cellular technologies operating on the same frequency band has become more and more interesting.
A typical goal with co-siting base stations has been to minimize the number of antennas and antenna feeders on the site. An antenna feeder corresponds here to a transmission line between antenna and radio relay equipment, transmitting the RF signal in either direction. Antenna feeders are typically very expensive, so it is natural that their number is optimized as far as possible. When cables are used, also the load on the mast structure needs to be considered. Additionally, in many locations the number of antennas per site is limited by regulations, due to the visual impact on the landscape.
On the other hand, a basic problem in digital radio communication is that a large number of errors occur due to channel attenuation, for example when the channel is in a fade. To overcome this, the base station receiver is typically provided with two or more replicas of the same information signal transmitted through independently fading channels. This means that the probability that all the signal components will fade simultaneously can be reduced considerably. The method of reception in which a single output signal is derived from a combination of or selection from a plurality of transmission channels or paths is called as diversity reception.
Sharing of antennas such that diversity reception is provided poses some technical requirements that need to be carefully considered. In the network planning phase the network configuration is assessed and dimensioned and a detailed network solution is defined. The basic parameters in dimensioning relate to system sensitivity of the base station and the output power available in the antenna. It is essential that these factors should not change considerably because of the sharing arrangement. It is also important that cost-effectiveness of the technical solution is considered from all aspects: the required equipment need to be inexpensive, easily installed and basically maintenance free. It is also desirable that only minimal changes to any existing site equipment be incurred, and the valuable radio band be used to the fullest possible extent.
One of the conventional ways to enable several radio receivers to utilize a single antenna system is to use multicouplers. There are many variations in the design concepts, but basically a multicoupler splits an input signal and distributes the partitioned RF signal. Wideband combining systems can be used for any frequency separation, which typically makes them very useful in site solutions. However, the theoretical loss introduced to the transmitted signal in wideband combining of signals from two base stations is about 3 dB, and in actual implementations reaches even 3,5 dB, at the minimum. In sharing, the sensitivity of the base station would also decrease correspondingly. Such degradation of the basic design criterion of the network planning is not acceptable, especially in the most typical sharing case, i.e. when a new base station is installed to share the feeder of another, already operating base station.
Filter combining is a method that may be used when a guard band exists between the channels, i.e. when a narrow frequency band is arranged between adjacent channels of the co-sited base stations. Such a guard band, however, leads to inefficient usage of the spectrum, which in some cases may even preclude the possibility of co-siting. Furthermore, in filter combining, a band pass filter passing only the operating frequencies of the respective base station is arranged between the feeder and the antenna connector of the base station. Even though the downlink power loss in filter combining is considerably smaller than in wideband combining, and the uplink power loss is to a great extent compensable with masthead amplifiers, the solution is too complicated and expensive for most of the installations. Firstly, the filters need to be tuneable to different channel allocations of different sites. This means that some kind of control system for controlling the parameters of the band pass filter would be needed, for example mechanical control with step motor. This increases the price of the filter components and thus the total cost of the sharing arrangement considerably. Additionally, the tuning needs to be done site by site, which complicates the commissioning and maintenance of the site.
In some prior art solutions, diversity is implemented by splitting the received signal with an internal divider located within the first base station and by forwarding the diversity reception signal to the other base station through an additional diversity cross feeding connector. In most modern integrated base stations such cross feeding dividers and connectors are typically not available, and introducing such arrangements to existing or new base stations is not economically viable. Additionally, the arrangement is not operable when the base stations are sub-banded, i.e. when the operable carrier block of the second base station does not coincide with the receiver carrier block of the first base station.